1. Field of the Invention
The present invention relates to an optical pickup device that is used when reading information from or writing information to an optical disc. More particularly, the invention relates to a technology suitable for an optical pickup device in which an objective lens that focuses light from a light source on an information recording surface of an optical disc is formed of resin.
2. Description of Related Art
Conventionally, reading information recorded on optical discs such as Blu-ray discs (BDs), and writing information to optical discs are performed by use of optical pickup devices. The optical pickup devices are provided with an objective lens that focuses light from a light source on an information recording surface of an optical disc. With regard to the objective lens, not a glass lens but a resin lens is conventionally used with a view to reducing the cost of the optical pickup devices (e.g., see Patent Document 1).
Resin lenses are easily affected by variation in ambient temperature; thus, if a resin lens is provided in the optical pickup devices as the objective lens, spherical aberration that occurs along with variation in ambient temperature may cause inconveniences. Note that the spherical aberration tends to cause inconveniences particularly in Blu-ray compatible optical pickup devices using a high NA objective lens.
Moreover, it has been said conventionally that the state of warping in an optical disc varies due to environmental changes (variation in temperature etc.) (e.g., see Patent Documents 2 and 3). When there is warping in the optical disc, disadvantageously, coma aberration may result in the optical pickup devices. In addition, when the warping state of the optical disc varies due to environmental changes, the amount of coma aberration produced varies; thus, the question is how to reduce the coma aberration in the optical pickup devices.
That is, in the optical pickup devices, proper reduction of aberration produced due to variation in ambient temperature is required.    Patent Document 1: JP-A-2003-287675    Patent Document 2: JP-A-7-282453    Patent Document 3: JP-A-2007-294023